1. Field of Invention
The present invention relates to a radiation spectrometer. More specifically, a radiation spectrometer of the present invention identifies, quantifies and accurately locates radioactive material.
2. Description of the Related Art
Locating and identifying radioactive sources is a common task in the areas of radiation protection, radioactive waste control, nuclear proliferation and homeland security. Gamma-rays penetrate surrounding materials without losing their characteristic energies so identifying the particular radioactive isotopes in the source is done by gamma-ray spectroscopy.
Most gamma-ray detectors give only a limited amount of position information. The amount of radiation that is detected from a point gamma-ray source reduces as the distance, d, increases according to the formula: one divided by the distance squared. This relationship between energy and distance causes the count rate to increase as the gamma-ray detector is placed nearer to the source, thus providing some position information. Some practical problems require the detector and/or the source to remain fixed in position so scanning the detector to locate the source can not be used. A typical example of such a measurement would be monitoring a pedestrian portal or border crossing, looking for illegal radioactive materials. Another example would be monitoring barrels of low level radioactive waste, looking for sources that require special handling for disposal.
Low level radioactive waste is usually contained in standard 55-gallon drums. The drum is placed on a measurement platform located at a fixed position relative to a gamma-ray spectrometer and a gamma-ray spectrum obtained. The energies measured in the spectrum identify the specific radioactive isotope. The count rate at specific energies is indicative of the amount of the isotope present. If the radioactive source happens to be located in the drum close to the detector the count rate will be relatively high. If the source is located further away, the count rate will be lower. Rotation of the barrel around its axis can be used to average out some of this variation. Moving the drum (or the detector) along the vertical axis of the drum can also give a result more indicative of the average radiation level.
A higher than normal indication of a dangerous isotope will require a measurement to more accurately locate the exact position of the source. Placing a collimator between the detector and the drum, so that the radiation seen by the detector can only come from a specific direction, gives position information but only by drastically reducing the counting efficiency. Reduced counting efficiency requires a longer counting time in order to estimate the intensity of the source. Making the collimator smaller improves the position information but reduces the counting efficiency. Making the collimator larger improves the counting efficiency but degrades the position information.